1. Field of the Invention
The present invention relates to a nitride-based semiconductor device, and a method for manufacturing the semiconductor device. More particularly, the present invention relates to a nitride-based semiconductor device in which a heterostructure field effect transistor (HFET) and a surface acoustic wave (SAW) filter are integrated on a single substrate, and a method for manufacturing the semiconductor device.
2. Description of the Related Art
With the recent development of communication technologies, there is an increasing need for high-speed and high-power electronic devices essential to ultrahigh-speed digital communication systems. For this reason, a number of studies on semiconductor materials which can accomplish high-speed and high-power operation characteristics are being actively undertaken. In particular, since GaN as a nitride-based semiconductor material has superior physical properties, e.g., large energy gap, superior thermal and chemical stability, high electron saturation velocity (˜3×107 cm/sec), etc., it has strong potential for application to not only optoelectronic devices but also high-frequency and high-power electronic devices. Thus, a great deal of research on GaN has been conducted in the fields of devices and processes.
Nitride-based semiconductor electronic devices manufactured by employing GaN have many advantages in terms of a high breakdown voltage, maximum current density, stable high temperature operation, high thermal conductivity, and the like. Since heterostructure field effect transistors (HFETs) fabricated using an AlGaN/GaN heterojunction structure have band-discontinuity at the junction interface, a 2-dimensional electron gas (2-DEG) layer where many free electrons are crowded is formed at the interface, thereby further increasing the electron mobility. Further, since the GaN layer has a high surface-acoustic-wave velocity, superior temperature stability and polarization effects of piezoelectricity, it can be easily used for the fabrication of a band-pass filter which can be operated on the order of GHz or more.
In connection with SAW filters in the GHz band, recent research has been concentrated on materials having a high SAW propagation velocity, process techniques for forming electrode patterns with a width below sub-micron level, and methods for fabricating SAW filters using a harmonic frequency mode. However, conventional SAW filters fabricated using LiNbO3, LiTaO3 or quartz use different semiconductor materials from amplification devices, such as HFETs and heterostructure bipolar transistors (HBTs), used in RF integrated circuits. Under this circumstance, the SAW filters and amplification devices cannot be mounted on one chip, and instead have been mounted in a hybrid mode.
To solve this problem, Korean Patent Laid-open No. 2004-0046479 suggests a method for integrating a SAW filter and an HFET on a single wafer using a semi-insulating GaN layer. According to this method, however, the surface of the semi-insulating GaN layer is impaired in the course of dry etching the GaN layer to form the SAW filter, deteriorating the surface characteristics of the SAW filter.
A conventional method for manufacturing an HFET and a SAW filter on a single substrate is illustrated in FIGS. 1a and 1b. 
Referring first to FIG. 1a, a semi-insulating GaN layer 12 and an AlGaN layer 13 are sequentially formed on a substrate 11. Thereafter, predetermined regions of the AlGaN layer 13 are etched by dry etching, e.g., reactive ion etching, and then electrodes 14a and 14b for an HFET are formed on the unetched regions of the AlGaN layer 13 (FIG. 1b). At this time, the etching is excessively conducted in such a manner that some portions of the semi-insulating GaN layer 12 are etched. Electrodes 15 for a SAW filter are then formed on predetermined regions of the exposed portions, as shown in FIG. 1c. Finally, as shown in FIG. 1d, the semi-insulating GaN layer 12 is etched to form trenches 16 for separating the devices, i.e., HFET and SAW filter, from each other.
According to the conventional method, the portions of the GaN layer 12 in the region where a SAW filter is to be formed are etched during dry etching of the AlGaN layer 13. As a result, the surface of the GaN layer 12, acting as a piezoelectric material of the SAW filter, is impaired by the dry-etching and thus the surface characteristics are deteriorated. In addition, since additional dry-etching is required to separate the HFET from the SAW filter, the process is complicated and there may be a risk of misalignment upon dry etching the GaN layer. Furthermore, since an AlGaN layer is required to be formed on the semi-insulating GaN layer containing a large number of crystal defects to directly form a heterostructure, crystallinity in the vicinity of the heterostructure is degraded, making the fabrication of an HFET having a high-mobility channel more difficult, and the amount of leakage current increases due to the presence of a number of defects.